Please unzip the package you got from themeforest.net ONE time. You should get a folder which contains PSDs, Docs and another zip file (which is called sentence.zip. This zip file is the right file. Go to your wordpress admin panel and navigate to Appearance > Themes. Then select the sentence.zip file, upload & install it. You can also unpack this zip file and upload the resulting folder via ftp to wp-content/themes.